1. Field of the Invention
The present invention relates generally to the field of solid state, active aperture array antennas for radar, and more particularly to apparatus and methods for reducing sidelobe radiation by such antennas.
2. Discussion of the Background
Radar antennas are well known to radiate microwave radiation in a braod pattern which, for a directed antenna, includes a narrow mainlobe and wide sidelobes of radiation. By common definition, the mainlobe is the central lobe of a directional antenna's radiation pattern, the sidelobes referring to the lesser lobes of progressively decreasing amplitude on both sides of the mainlobe and often extending rearwardly of the mainlobe.
Radar antenna aperture configuration generally determines the extent and relative magnitude of the associated sidelobes; however, the gain of the strongest one of the sidelobes is typically only about 1/64 that of the mainlobe. In terms of decibels, the strongest sidelobe gain is typically down about 18 dB from the associated mainlobe gain. Gains of the other sidelobes are usually considerably smaller than that of the strongest sidelobe. Although sidelobe gain is typically much smaller than mainlobe gain, because of the large solid angle into which sidelobes radiate, as compared to the small solid angle into which the mainlobe radiates, typically about 25 percent of the total power is radiated by a uniformly illuminated radar antenna in the sidelobes.
Ordinarily, sidelobe radiation provides no useful function and in addition to representing wasted radiating power has other serious disadvantages. For example, radar clutter from sidelobe returns increases the difficulty of discriminating targets from background. Another very significant disadvantage of sidelobe radiation is that such radiation can, in a military environment, be utilized by hostile forces for electronically jamming the radar and can also be used for positionally locating and for guiding munitions to the radar. In this regard, although mainlobe radiation is ordinarily much greater than sidelobe radiation, its relatively small solid angle of radiation and its directionality makes mainlobe jamming, radar location and munitions direction more difficult.
For these and other reasons, the reduction or suppression of radar sidelobe radiation is, particularly in military radar, important and military procurement documents establishing rigid limits on sidelobe radiation are not uncommon.
It is generally known that sidelobe radiation can be suppressed in array-type radar antennas by "tapering" the illumination over the aperture so that individual radiation-emitting elements near the side edges of the array radiate less energy than do other elements closer to the center of the array. Power may, for example, be individually applied to emitting elements of the array, so that the radiation energy distribution across the array, in at least one direction, is substantially Gaussian.
Radar arrays have, until quite recently, been "passive" types in which each radiating element in the array is provided power from a large, common power source. For such passive arrays, tapering of the radiation output, or, as it is sometimes termed, tapering of array illumination, is comparatively easy to implement by the use of restrictive branching from the power source to the radiating elements, such that progressively lower power is provided to elements further from the array center.
More recently, however, there has been great interest in developing active aperture arrays in which each radiating element, or a subgroup of elements, in the array is driven by a separate, small, solid state power supply or module. Active arrays have numerous actual and potential advantages over passive arrays. As an example, the power modules of the active arrays, being physically dispersed across the array, can be cooled more efficiently and effectively than the single, high power source of a corresponding passive array. Moreover, within a large active array, a comparative large number of power modules can fail or malfunction without substantially impairing effectiveness of the antenna. In contrast, failure or malfunction of the common power source in a passive array incapacitates the entire antenna.
According to theory, the providing of very smoothly tapered illumination of passive array antennas should be possible by the use of many (about 20 or more) different groups of power modules, each group having a different power output. In reality, however, the use of many different power groups of modules is not practical because such construction adds substantially to the cost of producing the arrays and causes subsequent maintenance and logistical support problems. As an illustration, if twenty different power modules groups were to be used in an array, supplies of all twenty different type modules would have to be stocked wherever any array maintenance and repair activities are expected to be needed.
As a result of costs and problems involved with using a large number of different power module groups in active arrays, sidelobe reduction has generally been attempted using only a relatively few different power module groups which have heretofore provided only coarsely tapered array illumination and relatively poor side lobe reduction. The selection of power module operating levels and their arrangement has, so far as is known to the present inventors, been previously made merely by approximately fitting the resulting, staircase-shaped distribution, having only a few steps, to an optimal distribution which may, for example, be in the bell-shape of a Gaussian distribution. Such fitting of an actual, stepped distribution to an optimum distribution curve has not heretofar, also so far as is known to the present inventors, been based upon any rigorous, systematic analysis and has not, therefore, except possibly in isolated, accidental cases, resulted in minimal sidelobes. Nor have such heretofore used curve-fitting approaches enabled specific sidelobe radiation levels to be predicted or designed to, as is often required to meet procurement specifications.
As a result, to satisfy present and anticipated future low sidelobe requirements for solid state active array antennas, improvements are required in the design of such antennas, and specifically in processes for the systematic selection of power module operating levels and physical arrangements of power modules operating at different power levels so as to provide low sidelobes. It is to such a systematic approach for power module operating levels and arrangements that the present invention is directed.